This invention pertains to noise reduction for centrifugal fans, and particularly pertains to a rotary cutoff device attachable to such fans and method for reducing noise pulsations produced by operation of the fan.
Centrifugal fans are widely used in steam generator/power plant systems, process plants, mines, tunnel ventilation and other systems having requirements of moving large quantities of air or gases. It is known that these fans are inherently quite noisy, and special efforts have been exerted to reduce the emitted fan noise to environmentally acceptable levels. The standard noise reduction methods used include extensive sound insulation of the fan housing, sound insulation of the fan upstream and downstream ducting, and installation of fan inlet and discharge silencing equipment. However, such noise reduction methods add significantly to the fan installation costs, and the inlet and discharge silencers introduce pressure drop losses which lead to increased power consumption.
Because of these deficiencies in the known arrangements for noise reduction for centrifugal fans, various other noise reduction means have been developed. For example, U.S. Pat. No. 3,712,412 to Hassett discloses a sound suppressing system for high velocity fluid streams such as from high speed fans by using tunable resonating chambers. U.S. Pat. No. 4,174,020 and U.S. Pat. No. 4,279,325 to Challis disclose stationary sound insulation apparatus having cavities for acoustic treatment for centrifugal fans. Also, U.S. Pat. No. 4,475,621 to Cherington et al disclose use of acoustic sound reduction chambers for a combustion furnace. However, further improvements are needed for effective reduction of undesirable noise produced by large operating fans such as from centrifugal fans.